1. Field of the Invention
This invention relates to an integrated photovoltaic device, particularly relates to a manufacturing method of a photovoltaic device provided with a transparent electrode of zinc oxide.
2. Description of Prior Art
A photovoltaic device such as a solar cell can directly convert sunlight into electricity and has been commercialized as a new energy source. Such the photovoltaic device has been formed of crystalline semiconductor material such as single crystalline silicon and polycrystalline silicon, compound semiconductor material such as GaAs, InP or the like, and amorphous semiconductor material such as amorphous silicon and amorphous silicon germanium or the like. The photovoltaic device using the amorphous semiconductor material is manufactured at a low temperature as compared with other semiconductor material, can increase a size easily, and can easily be integrated on a substrate.
FIG. 1 is a structural cross sectional view of an integrated photovoltaic device using the amorphous semiconductor material.
Explanation is made on the integrated photovoltaic device by referring to FIG. 1.
A substrate 1 is formed of translucent and insulating material such as glass, plastic, or the like. A plurality of first electrodes 2 are arranged on a surface of the substrate 1. The first electrode 2 is formed of tin oxide (SnO2) and has a rough plane for scattering light incident from the substrate 1 on the surface. This rough plane is generally referred as a texture plane.
A photovoltaic conversion layer 3 is formed of amorphous semiconductor material, and generally includes a p-type amorphous silicon carbide film of approximately 100 xc3x85, an intrinsic amorphous silicon film of approximately 4000 xc3x85, and an n-type amorphous silicon film of approximately 200 xc3x85 laminated in this order from a side of the first electrode 2. A second electrode 4 is formed of a highly reflective metal film such as Ag, Al or the like.
A lamination body of the first electrode 2, the photovoltaic conversion layer 3, and the second electrode 4 is a unit cell 10, and adjacent unit cells 10 are electrically connected in series by electrically linking the first electrode 2 of the one unit cell 10 and the second electrode 4 of the other unit cell 10.
FIGS. 2A-2E are structural cross sectional views of each of processes for illustrating manufacturing processes of the conventional photovoltaic device.
As shown in FIG. 2A, a transparent electrode film 21 of tin oxide (SnO2) having a texture plane is formed on a surface of the substrate 1. A predetermined part of the transparent electrode film 21 is eliminated by laser beam irradiation and is divided into a plurality of the first electrodes 2, as shown in FIG. 2B.
An amorphous semiconductor film 31 having pin junction inside is formed on the substrate so as to cover over the first electrodes 2 as shown in FIG. 2C.
Then, a predetermined part of the amorphous semiconductor film 31 is eliminated by laser beam irradiation and is divided into a plurality of the photovoltaic conversion layers 3 as shown in FIG. 2D.
A metal film 4 is formed on the substrate 1 so as to cover the photovoltaic conversion layers 3 as shown in FIG. 2E. Then, a predetermined part of the metal film 41 is eliminated by laser beam irradiation and is divided into a plurality of the second electrodes 4 to produce the photovoltaic device as shown in FIG. 1.
The material for forming the first electrode as the transparent electrode is conventionally SnO2. However, formation of SnO2 requires a temperature higher than approximately 500xc2x0 C., resulting in an increase in manufacture cost. And a substrate of less heat resistance such as plastic can not be usable and selection of substrate material is limited.
In conjunction with this, zinc oxide as material for the first electrode has been considered. The first electrode of zinc oxide can be prepared at a low temperature of not higher than 200xc2x0 C. by sputtering, resulting in reduction of manufacture cost. In addition, selection of the substrate material is not limited as tightly as in the conventional case.
The applicants of this invention have examined and found that it is more difficult to electrically separate the adjacent first electrodes of zinc oxide in eliminating the predetermined part of the zinc oxide film by laser beam irradiation and dividing into a plurality of the first electrodes as compared with the first electrode of a SnO2 film. Thus, leak current is likely to occur between adjacent unit cells of the photovoltaic device comprising the first electrodes of zinc oxide and the photovoltaic conversion characteristics are degraded.
A cause of the above problem has been examined. An expected cause of the problem is explained by referring to schematic cross sectional views of FIGS. 3A-3B. Elements having the same functions as in FIG. 1 have the same reference numerals.
When laser beam intensity for irradiating to the zinc oxide film is great, the temperature of the zinc oxide film increases and doping material of Al, Mg, Ga or the like for reducing a resistance value included in the zinc oxide film is scattered in the substrate 1 to form the diffusion region 1A on a surface of the substrate as shown in FIG. 3A. And leak current is generated between the adjacent first electrodes 2, 2 through the diffusion region 1A.
On the other hand, when laser beam intensity is small so as to suppress formation of the diffusion region 1A, the residual 21A of the zinc oxide film 21 is generated as shown in FIG. 3B and leak current occurs between the adjacent first electrodes 2 through the residual 21A.
When the first electrode is formed of zinc oxide and intensity of laser beams to be irradiated is great, doping material of Al, Mg, Ga or the like included in the zinc oxide is scattered and the diffusion region is formed on a surface of the substrate. When the intensity of the laser beams is small, residual of zinc oxide is generated. In these cases, it is expected that leak current occurs between the adjacent first electrodes through the diffusion region and the residual, and photovoltaic conversion characteristic are degraded.
In recent years, a thin film semiconductor device using a glass substrate has become large. A solar cell device which is used in the outside is particularly required to have mechanical strength. In conjunction with this, only attachment of a reinforced glass which requires complicated processes and is difficult to reduce production cost, and a reinforcement process, as a post-process, of a glass substrate with a transparent electrode which is difficult to have sufficient strength and film characteristics have been considered. In this case, when a temperature of the reinforced glass increases higher than 500xc2x0 C. of a glass melting point after the reinforcement process, an effect of reinforcement and, as a result, the strength may be degraded. Although a SnO2 film is commonly used material, this film can have sufficient characteristics only when formed at higher than 500xc2x0 C. Therefore, the reinforced glass can not be used as a substrate. Thus, a method for reinforcing a glass substrate after mounting a transparent electrode has been examined.
In processing the transparent electrode by energy beams, the energy beams should be applied so that the temperature increases momentarily up to approximately 2000xc2x0 C. of a melting point. Therefore, heat from irradiation energy transmits to a side of the glass when the energy beams are irradiated in order to divide the transparent electrode into a plurality of regions even though the transparent electrode of good characteristics are formed at lower than 500xc2x0 C. As a result, the temperature increases up to higher than 500xc2x0 C. locally, which causes fine cracks and irregular degradation of strength of the part of the glass to degrade the strength of the entire glass.
This invention was made to solve the existing problem and provides a manufacturing method of a photovoltaic device comprising a first electrode of zinc oxide capable of improving photovoltaic conversion characteristics.
This invention provides a manufacturing method capable of using reinforced glass as a substrate, improving photovoltaic conversion characteristic, and having sufficient mechanical strength.
A manufacturing method of a photovoltaic device provided with a plurality of unit cells including a first electrode of zinc oxide, a photovoltaic conversion layer, and a second electrode on a surface of a transparent substrate, comprises a process for forming a zinc oxide film on a surface of a substrate, a process for eliminating a predetermined part of the zinc oxide film by energy beam irradiation and dividing the zinc oxide film into a plurality of the first electrodes, and a process for etching the surface of the substrate provided with the plurality of the first electrodes.
A texture plane is formed on a surface of the first electrode by etching.
In this invention, a predetermined part of the zinc oxide film is eliminated by energy beam irradiation and the zinc oxide film is divided into a plurality of the first electrodes. Then, a diffusion region and residual of zinc oxide formed on a surface of the substrate are eliminated by etching. Leak current between the adjacent first electrodes decreases greatly as compared with the conventional case. Therefore, a photovoltaic device with excellent photovoltaic conversion characteristics can be provided by this invention.
A manufacturing method of a photovoltaic device provided with a plurality of unit cells including a first electrode of zinc oxide, a photovoltaic conversion layer, and a second electrode on a surface of a transparent substrate, comprising a process for forming a zinc oxide film on a surface of a substrate, a process for eliminating a predetermined part of the zinc oxide film by energy beam irradiation so as to leave the zinc oxide film of not less than a predetermined thickness on a side of the transparent substrate and preliminarily dividing the zinc oxide film into a plurality of the first electrodes, and a process for etching the surface of the substrate provided with the plurality of the first electrodes which are preliminarily divided, and electrically separating adjacent divided areas.
The transparent substrate is a reinforced glass.
In this invention, the transparent electrode of more than a predetermined thickness is intentionally remained on a glass substrate side of the transparent electrode in a process for dividing into a plurality of regions by energy beams, and the adjacent divided regions are electrically separated in the etching process. Degradation of substrate strength caused by fine cracks and irregular degradation of strength caused by heat transmitted to the glass in processing the transparent electrodes by energy beams is prevented with these processes.
In addition, development of over-etching is prevented in a readily etched part between the substrate and the transparent electrodes when attachment of scattered material generated in processing the transparent electrodes by energy beam prevents complete separation of the transparent electrodes. Thus, peeling of a transparent conductive film in proximity of a processed part is prevented and reliability and performance of a device are improved.
The above second constitution prevents an increase of a temperature to a melting point of glass and maintains an effect of reinforcement, thus is particularly effective in using reinforced glass.